In air jet looms several types of weft thread may be used for obtaining special fabrics and fabric effects. Each weft thread is pulled off its respective weft thread supply spool and taken over by weft thread insertion nozzles, including a main nozzle located at a weft thread insertion side of the loom shed and a plurality of auxiliary nozzles arranged alongside the weft thread insertion channel formed by the reed and passing through the loom shed. The nozzles are controlled by a central control unit including air insertion control means to provide the proper required air stream in the air channel formed by profiled teeth of the reed. A weft thread monitor is arranged at the weft thread exit side of the weft thread insertion channel for monitoring the presence or absence of a weft thread. A control signal is produced, for example, to stop the loom in response to a broken weft thread. The weft thread monitor comprises monitoring elements such as light emitting and light sensing elements which produce the mentioned control signal which is supplied to the input of an amplifier.
Conventional weft thread monitors of this type are capable to some extent to distinguish yarn qualities, however, a precise distinction or rather high resolution is not possible with conventional weft thread monitors because thick coarse threads and very fine threads cannot be distinguished from each other. Pseudo-faults such as harmless lint or fluff particles passing through the monitor may cause a false shut-off signal. Such monitoring errors may be made quite frequently by conventional monitors.
In a situation in which a conventional weft thread monitor is supposed, for example, to recognize a very fine weft thread yarn, it is necessary to provide a high amplification factor or gain for the signals received from a light barrier through which the fine weft thread is passing. On the other hand, a substantially smaller amplification or gain is required where a weft thread of substantial thickness is being monitored because such a thicker weft thread can be optically monitored much more easily. In other words, in conventional weft thread monitors the output signal of the monitoring elements depends very much on the mechanical characteristics of the weft thread yarn so that a thick yarn is easily monitored while a very fine or thin yarn cannot be sufficiently monitored or sensed. However, increasing the amplification when a fine weft thread yarn is monitored can cause the problem that the amplification is much too high when the yarn thickness suddenly changes. As a result, even a yarn slub or fluff particle can cause the generation of a false warning signal by the weft thread monitor. Substantial weaving errors or flaws can be the result of such conventional weft thread monitoring. Conventional monitors of this type, in addition to not being able to react equally to thick and thin weft threads, also cannot well respond to different air effective qualities of different types of weft thread yarns. The term "air effective qualities" refers to thread characteristics which have an effect on the movement of the weft thread through the air and on the movability of the thread by the inserting air jet. For example, a coarse, hairy yarn with a rough surface responds differently to the transporting air jet than a yarn with a smooth surface. Similarly, a coarse yarn produces more lint balls or fluffs than a fine smooth surfaced yarn. Neither different mechanical yarn qualities nor the air effective yarn qualities must cause a false shut-down signal. Conventional monitors of this type leave room for improvement in this respect.
The mechanical yarn qualities such as the yarn thickness and the air effective yarn qualities together are referred to as yarn specific values in the following text.